1. Field of the Invention
The present invention relates to a method of and an apparatus for drilling a printed circuit board by means of a drill.
2. Description of the Related Art
In general, printed circuit boards are drilled by using a drill.
For example, the essential portion of an apparatus for drilling printed circuit boards is constructed as shown in FIG. 1. (For further detail, refer to commonly assigned Japanese Patent Application Laid-Open No. 205209/1984).
As illustrated, a chuck 101a is supported at one end of a spindle 101. A cylinder 103 is disposed for movement along its axis and supports the spindle 101 for rotation about its, axis. A slide portion 103a is formed between the spindle 101 and the cylinder 103. A piston 104 is supported by the slide portion 103a in such a manner that it can move along the axis of the spindle 101. A space 115 defined by the spindle 101, the cylinder 103 and the piston 104 is supplied with compressed air under fixed pressure from a compressed-air source (not shown) through a fluid conduit 116.
A pressure foot 5 having a hole 6 connected to a dust collector is fixed to one end of the piston 104.
A drill 4 is supported by the chuck 101a.
A plurality of printed circuit boards 1 which are to be drilled by such an apparatus for drilling printed circuit boards are stacked. The stack of the printed circuit boards 1 is sandwiched between an entry board 2 and a back-up board 3, and they are integrally fixed.
As shown in FIG. 2, when the spindle 101 is moved toward the stacked printed circuit boards 1, the pressure foot 5 first comes into contact with the entry board 2. When the spindle 101 is further moved toward the stacked printed circuit boards 1, the entry board 2, the stacked printed circuit boards 1 and the back-up board 3 are pressed by the pressure foot 5 under the pressure supplied to the space 115 since the movement of the pressure foot 5 is restricted by the contact with the entry board 2.
When, in this state, the spindle 101 is further moved toward the stacked printed circuit boards 1, the tip of the drill 4 engages with the entry board 2 and initiates drilling. Subsequently, the spindle 101 is moved to a point at which the tip of the drill 4 penetrates into the back-up board 2, thus completing drilling of the stacked printed circuit boards 1.
In such a drilling method, if one back-up board 3 having a thickness of 0.063 inches, three printed circuit boards 1 each having a thickness of 0.063 inches and one entry board 2 having a thickness of 0.04 inches are integrally fixed and a hole is drilled to a depth of 0.04 inches in the back-up board 3 from the surface thereof opposing the entry board 2, the total depth of the hole is 0.269 inches.
If the diameter of a hole to be drilled is, for example, 0.047 inches, the resultant aspect ratio (the ratio of the depth Lh of the hole and the diameter D) is 5.7.
When the aspect ratio is relatively small (not greater than 6) as described above, the drill 4 can be fed from a drill-starting position A to a drill end position B to drill the stacked printed circuit boards 1.
However, if the diameter D of a hole to be drilled is, for example, 0.031 inches, the resulting aspect ratio is 8.7.
When the aspect ratio is large as described above, it is impossible to effect drilling by feeding the drill 4 from the drill-starting position A over to the drill end position B in one step.
In other words, when the aspect ratio is excessively large, it is difficult to eliminate debris produced during drilling and such debris clogs the flute of the drill 4. As a result, the following problems may occur:
1. The surface roughness of the inner circumferential wall of a drilled hole increases.
2. The amount of heat generated due to friction during drilling becomes large and the temperature of the drill 4 rises to an excessive degree. Therefore, the thermal wear of the drill 4 increases.
3. The base material (for example, an epoxy resin) of each printed circuit board 1 melts due to the temperature rise in the drill 4, and thus even larger amounts of smears stick to, and hardens on, the end surface of a copper foil exposed in the inner circumferential wall of the drilled hole.
4. Further, a load applied to the drill 4 increases, and the drill 4 may be easily broken.
Accordingly, a proposal has been made to make a hole having a large aspect ratio, as shown in FIG. 3.
For example, the distance between the drill-starting position A and the drill end position B is divided into three stages with intermediate positions M1 and M2 set partway down the hole.
The feed rate of the drill is selected between a drilling feed rate VF and a rapid feed rate VR.
First, the drill is fed from the drill-starting position A to the first intermediate position M1 at the drilling feed rate VF. Then, the drill is fed from the intermediate position M1 to the drill-starting position A at the rapid feed rate VR. Subsequently, the drill is fed from the drill-starting position A to the intermediate position M1 at the rapid feed rate VR. When the tip of the drill reaches the intermediate position M1, the feed rate of the drill is switched from the rapid feed rate VR to the drilling feed rate VF. Then, the drill is fed from the intermediate position M1 to the second intermediate position M2, then from the intermediate position M1 back to the drill-starting position A at the rapid feed rate VR. Then, the drill is fed from the drill-starting position A to the intermediate position M2 at the rapid feed rate Vx. When the tip of the drill reaches the intermediate position M2, the feed rate of the drill is switched from the rapid feed rate VR to the drilling feed rate Vr. Then, the drill is fed from the intermediate position M2 to the drill end position B. After the drill has reached the drill end position B, the drill is fed from the drill end position B back to the drill-starting position A at the rapid feed rate VR.
Since the drill is fed reciprocally between the drill-starting position A and the intermediate positions M1, M2 to effect drilling, the following advantages can be obtained:
1. Chips filling the drill flute are shaken off when the drill is located out of the entry board. Therefore, the problems caused by chip clogging are eliminated.
2. The drill is cooled when the drill is located out of the entry board. Therefore, it is possible to reduce the thermal wear of the tip of the drill.
Such a method of drilling a hole by reciprocally feeding a drill is disclosed in, for example, U.S. Pat. Nos. 2,768,539, 3,129,613, 3,637,318, and 4,123,188.
However, none of the above patents disclose the use of the pressure foot required to drill a hole in a printed circuit board.
In other words, none of the above prior arts suppose drilling of soft workpieces such as printed circuit boards.
In general, in the case of drilling printed circuit boards, the entry board or the printed circuit boards exhibit softness. The pressure foot is connected to a dust collector and a vacuum is produced in the space in the pressure foot during drilling. Thus, the space is always maintained at a negative pressure during drilling.
Accordingly, each time the spindle is fed upwardly, the entry plate is pulled upwardly by the negative pressure in the pressure foot. Therefore, stresses due to the repetition of the upward pulling and recovery are produced in the entry plate and, while a multiplicity of holes are being drilled, wear occurs in the fixed parts of the entry plate and the entry plate may be displaced horizontally. If the entry plate is displaced, the position of the hole which has been formed in the entry plate when drilling has reached the initial intermediate position M1 is deviated from the entry position of the drill when the next drilling to the intermediate position M2 is initiated Accordingly, if the drill is reciprocally fed by a plurality of times during drilling of one hole, the number of times by which the entry plate makes contact with the drill increases, and the amounts of wear and bending of the drill increase. As a result, the following problems shown in FIGS. 4A to 4D will be encountered:
1. The positional accuracy of the hole and the configuration of the hole is degraded (refer to FIGS. 4A and 4C).
2. The amounts of smears sticking to the inner wall of the hole increases (refer to FIG. 4B).
3. The extent of deformation of the copper foil of the printed circuit board before drilling increases (refer to FIG. 4D).
For example, U.S. Pat. Nos. 4,212,570, 4,340,326 disclose the use of such a pressure foot.
However, none of the last-mentioned patents disclose the method of effecting drilling by feeding the drill reciprocally.
In other words, these patents suppose that a hole is drilled in stacked printed circuit boards in one step.
As is well known, the packaging density of electronic parts on a printed circuit board has been increasing. In such a situation, it is desired that the line width of a wiring pattern formed on a printed circuit board be reduced to 0.04 inches or less. Accordingly, it is demanded that the diameters of holes formed in the printed circuit board be made extremely small, for example, 0.016 inches or less. As a result, an increase in an aspect ratio cannot be avoided and it has become difficult to complete drilling in one step.
Since a drilling method such as that shown in FIG. 3 involves the step of feeding the drill from the drill end position B back to the drill-starting position A for the purpose of achieving reciprocal feeding of the drill, it has been impossible to avoid the problem that the time period required to drill one hole becomes long and the efficiency of working is consequently deteriorated.
For example, there is a case where several thousands to some tens of thousands holes are drilled in a single printed circuit board. In such a case, if the time period required to drill each hole is extended by 0.1 seconds, then the total time period for drilling each printed circuit board becomes longer by several minutes to several hours.
In general, printed circuit boards are manufactured through a multiplicity of steps such as (1) drilling, (2) through-hole copper plating, (3) patterning, (4) etching and stripping, (5) printing for solder resist, (6) gold plating on contact fingers, (7) solder coating, (8) press for blanking and finishing, (9) electrical inspection, (10) final inspection, (11) flux coating, (12) shipment, and (13) mounting. The drilling step is the first step in the process for manufacturing printed circuit boards.
Accordingly, since an error in drilling and the quality of a drilled hole may influence all the subsequent steps, the development of high precision and high grade drilling is demanded.